Method for the purification of N-[N-(3,3-Dimethylbutyl)-L-a-Aspartyl]-L-Phenylalanine 1-Methyl Ester

ABSTRACT

This invention relates to the purification of N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester by slurrying a mixture containing N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester with organic or aqueous organic solvents.

[0001] This application claims the benefit of U.S. Provisional Patentapplication Ser. No. 60/145,935, filed Jul. 28, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to an improved method for the purificationof an N-alkylated aspartame derivative, which is a particularly usefulsweetening agent.

[0004] 2. Related Background Art

[0005] N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methylester (neotame) is a high potency dipeptide sweetener (about 8000Xsweeter than sucrose) that has the formula

[0006] The synthesis of neotame is disclosed in U.S. Pat. No. 5,480,668,U.S. Pat. No. 5,510,508 and U.S. Pat. No. 5,728,862, the disclosure ofeach of which is incorporated by reference herein. These processes,however, may produce several impurities, includingN,N-di(3,3-dimethylbutyl)-L-aspartyl-L-phenylalanine methyl ester(dialkylated aspartame), α-methylhydrogen-3-(3,3-dimethylbutyl)-2-L-(2,2-dimethylpropyl)-5-oxo-α-L-(phenylmethyl)-1,4(L)-imidazolidinediacetate (dialkylated imidazolidinone),N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine (demethylated α-or β-neotame) and methyl ester ofN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester(methylated α- or β-neotame). These impurities are representedrespectively by the structural formulae:

[0007] The primary impurities generated in the synthesis ofN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester,i.e., dialkylated aspartame, methyl ester of α-neotame and dialkylatedimidazolidinone, have very similar solubilities compared toN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl esterand thus are difficult to remove by crystallization, thereby making itdifficult to obtain pure product in high yield.

[0008] Since neotame is mainly employed in foods for human consumption,it is extremely important that neotame exist in a highly purified state.Thus, it is clear that there is a need to economically produce pureN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.

SUMMARY OF THE INVENTION

[0009] This invention is directed to a process of purifying[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester. Theprocess comprises the steps of: (a) washing neotame with water; (b)combining the water washed neotame with an organic solvent to form aslurry; and (c) filtering the slurry to recover purified neotame. Theorganic solvent of step (b) may be an aqueous organic solvent.

[0010] Preferably, the process includes the steps of washing the neotamerecovered in step (c) with an organic solvent, which may be the same ordifferent than the organic solvent of step (b) and drying the washedfiltered slurry to recover further purified neotame.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIGS. 1 and 2 are the powder x-ray diffraction patterns of thepurified neotame obtained by the methods of Examples 1 and 2,respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The aforementioned process purifies by removing dialkylatedaspartame, methylated α-neotame, demethylated α-neotame, demethylatedβ-neotame, aspartame and dialkylated imidazolidinone impurities fromneotame from post-hydrogenation mixtures or from mother liquors ofcrystallized neotame.

[0013] As described herein the step of washing neotame with water can becarried out in any way that allows for the contact of neotame withwater. The water may contain water miscible solvents, e.g. alcohols,acetonitrile, acetone and the like, in an amount up to about 5-15% byweight. The water wash can be achieved for example, by adding solidneotame to the water, or adding water to an organic solvent solution ofneotame and then drying off the organic solvent. The wash may be a batchwash, continuous wash, or the like. For example, the solid neotame maybe stirred with water and filtered. This step of the process reducesdemethylated α-neotame and demethylated β-neotame, which are moresoluble in water compared to neotame.

[0014] Generally, the water wash is conducted at a temperature in arange of about 10° C. to about 50° C. for a time of about 0.5 to about30 hours. After the water wash, the neotame is typically separated fromthe water by filtration or centrifugation. If desired the recoveredneotame may be dried in a vacuum oven, tumble dryer, or cone dryer at atemperature of about 25°-50° C.

[0015] The slurry of neotame and organic solvent is formed by combiningthe solvent and neotame in any desirable manner for a specific time. Itis preferable to stir the mixture. Typically, the mixture is held at atemperature of about 15° C. to about 45° C. for a time of about 0.5 toabout 30 hours. The slurry is then filtered or centrifuged to recoverthe purified neotame.

[0016] If desired a second slurry may be formed using the same ordifferent organic solvent followed by filtration or centrifugation. Therecovered neotame is then preferably dried at a temperature of about 30°C.-50° C. and a pressure of about 10-250 mm Hg.

[0017] Exemplary organic solvents employed in the process of thisinvention include ethyl acetate, diethyl ether, tert butyl methyl ether,butyl acetate, methyl acetate, propyl acetate, isopropyl acetate,cyclohexane, hexane, heptane, pentane, acetone, acetonitrile, methanol,ethanol, isopropanol, butanol, dichloromethane or chloroform. Preferablythe organic solvents employed are tert butyl methyl ether or ethylacetate.

[0018] As previously noted the water washedN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl estermay be slurried with an aqueous organic solvent. Other organic solventsthat may be used in the process of this invention include, withoutlimitation: tert butyl methyl ether (MTBE), diethylether,acetone/hexane, acetonitrile/hexane, acetonitrile/heptane,acetonitrile/pentane, methanol/heptane, ethanol/hexane,isopropanol/hexane, methanol/hexane, butanol/hexane,dichloromethane/hexane or chloroform/hexane.

[0019] Preferably, theN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl esterstarting material purified by the process of this invention is obtainedusing the techniques and procedures described in U.S. Pat. No.5,510,508, U.S. Pat. No. 5,480,668 U.S. Pat. No. 5,782,862 and inco-pending application, U.S. Provisional appln. Ser. No. 60/110,011, thedisclosures of each of which are incorporated by reference herein.

[0020] A preferred embodiment of this invention is directed to a processof purifying N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine1-methyl ester (neotame) by a process comprising the steps of (i)filtering an organic solvent preferably a methanol solution containingN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester tocreate a filtrate; (ii) adding water to the filtrate; (iii) removing theorganic solvent from the filtrate to precipitate solids, preferablyunder reduced pressure with stirring; (iv) filtering the precipitatedsolid; (v) washing the precipitated solid with water and drying toobtain water-washed neotame; (vi) mixing the water-washed neotame withan organic solvent, which may be the same or different than the organicsolvent of step (i) to create a slurry; (vii) filtering the slurry;(viii) washing the filtered slurry with an organic solvent which may bethe same or different than the organic solvent of step (vi); and (ix)drying the slurry to obtain purified neotame.

[0021] As used herein, the term “solution” refers to anyN-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methylester-containing solution. This includes, without limitation,hydrogenation reaction mixtures, crystallization mother liquors, washliquors and waste streams.

EXAMPLE 1

[0022] The hydrogenation mixture (120 mL) containing 10% (by wt.) ofneotame in methanol was filtered through a one inch celite bed on asintered glass funnel. Water (60 mL) was added to the filtrate andmethanol was removed under reduced pressure at 35° to 40° C. The mixturewas stirred at room temperature for 2-16 hours. The precipitated solidwas filtered, washed with water (100 ml) and dried at 35° to 40° C. for16-24 hours to get 80-85% of crude neotame. This product had 7% ofimpurities (dialkylated aspartame, methylated α-neotame, demethylatedα-neotame and dialkylated imidazolidinone). Ten grams of crude neotamewas stirred with 80 mL of tert butyl methyl ether (MTBE) at 22°-26° C.for 1 hour. The slurry was filtered, washed with 20 mL of MTBE and 50 mLof water, and dried at 37°-40° C. for 12-24 hours to get 8.8-9.1 g ofpure neotame. This product had a total of about 0.1% wt/wt ofdialkylated aspartame, methylated α-neotame, demethylated α-neotame anddialkylated imidazolidinone by HPLC. FIG. 1 shows the powder x-raypattern of the purified neotame obtained through this method.

EXAMPLE 2

[0023] The hydrogenation mixture (120 mL) was filtered through a oneinch celite bed on a sintered glass funnel. Water (60 mL) was added tothe filtrate and methanol was removed under evaporated under reducedpressure at 35°-40° C. The mixture was stirred at room temperature for4-16 hours. The precipated solid was filtered, washed with water anddried at 35°-40° C. for 16-24 hours to get 80-85% of crude neotame. Thisproduct contained ˜10% impurities (dialkylated aspartame, methylatedα-neotame and dialkylated imidazolidinone). Ten grams of crude neotamewas stirred with 14 mL of ethyl acetate at 22°-26° C. for 2 hours. Theslurry was filtered and washed with water (10 mL). The wet product wasdried at 40°-45° C. for 12-24 hours to get 6.8 g of pure neotame. Thisproduct had a total of 0.2% of dialkylated neotame, methylatedα-neotame, and dialkylated imidazolidinone by HPLC. FIG. 2 shows thepowder x-ray pattern of the purified neotame obtained through thismethod.

[0024] Other variations or modifications, which will be obvious to thoseskilled in the art, are within the scope and teachings of thisinvention. This invention is not to be limited except as set forth inthe following claims.

We claim:
 1. A process for purifying N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ether comprising the steps of: (a) washing N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester with water; (b) combining the water washed N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester with an organic solvent to form a slurry; and (c) filtering the slurry to recover purified N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.
 2. The process according to claim 1, wherein the water wash is conducted at a temperature in a range of about 10° C. to about 50° C.
 3. The process according to claim 1, wherein the water wash is conducted for a time of about 0.5 to about 30 hours.
 4. The process according to claim 1, wherein the slurry is held at a temperature of about 15° C. to about 45° C.
 5. The process according to claim 1, wherein the slurry is held for a time of about 0.5 to about 30 hours.
 6. The process according to claim 1, wherein said organic solvent of step (b) is selected from the group consisting of tert butyl methyl ether, ethyl acetate, diethyl ether, methyl acetate, butyl acetate, propyl acetate, isopropyl acetate, cyclohexane, acetone, hexane, acetonitrile, heptane, pentane, ethanol, methanol, isopropanol, butanol, dichloromethane, chloroform and mixtures thereof.
 7. The process according to claim 1, wherein said organic solvent of step (b) is an aqueous organic solvent comprising water and an organic solvent selected from the group consisting of tert butyl methyl ether, ethyl acetate, diethyl ether, methyl acetate, butyl acetate, propyl acetate, isopropyl acetate, cyclohexane, acetone, hexane, acetonitrile, heptane, pentane, methanol, ethanol, isopropanol, butanol, dichloromethane, chloroform and mixtures thereof.
 8. The process according to claim 6, wherein said organic solvent is a mixture of organic solvents selected from the group consisting of acetone/hexane, acetonitrile/hexane, acetonitrile/heptane, acetonitrile/pentane, ethanol/hexane, methanol/hexane, methanol/heptane, isopropanol/hexane, butanol/hexane, dichloromethane/hexane, and chloroform/hexane.
 9. The process according to claim 1, further comprising the step of drying said recovered N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.
 10. The process according to claim 9, wherein said step of drying is conducted at a temperature from about 25° C. to about 50° C.
 11. The process according to claim 10, wherein said step of drying is conducted at a pressure of 0.1 to 200 mm Hg.
 12. The process according to claim 1, further comprising the step of (d) washing the N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester recovered in step (c) with an organic solvent.
 13. The process according to claim 12, wherein said organic solvent used in step (d) is selected from the group consisting of tert butyl methyl ether, ethyl acetate, diethyl ether, methyl acetate, butyl acetate, propyl acetate, isopropyl acetate, cyclohexane, acetone, hexane, acetonitrile, heptane, pentane, methanol, ethanol, isopropanol, butanol, dichloromethane, chloroform and mixtures thereof.
 14. The process according to claim 12, further comprising the step of (e) drying the N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester washed with the organic solvent.
 15. The process according to claim 14, wherein said step of drying is conducted at a temperature from about 25° C. to about 50° C.
 16. The process according to claim 15, wherein said step of drying is conducted at a pressure of 0.1 to 200 mm Hg.
 17. A process of purifying N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester comprising: (i) filtering a methanol solution containing N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester to form a filtrate; (ii) adding water to the filtrate; (iii) removing the methanol from the filtrate and forming a solid precipitate; (iv) filtering the precipitated solids; (v) washing the precipitated solids with water and drying to obtain crude N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester; (vi) mixing said crude N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester with an organic solvent to create a slurry; (vii) filtering the slurry; (viii) washing the filtered slurry with the organic solvent; (ix) drying the slurry to obtain purified N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.
 18. The process according to claim 17, wherein the methanol was removed under reduced pressure.
 19. The process according to claim 17, wherein the methanol was removed at a temperature of about 20° C.-40° C.
 20. The process according to claim 17, wherein the said solid precipitate is formed with stirring at room temperature for about 2-24 hours.
 21. The process according to claim 17, wherein the mixture in step (v) is dried at about 25° C.-50° C.
 22. The process according to claim 17, wherein the mixture in step (v) is dried for about 5-48 hours.
 23. The process according to claim 17, wherein the organic solvent added in step (viii) is selected from the group consisting of: tert butyl methyl ether, ethyl acetate, diethyl ether, methyl acetate,butyl acetate, propyl acetate, isopropyl acetate, cyclohexane, acetone, hexane, acetonitrile, heptane, pentane, ethanol, methanol, isopropanol, butanol, dichloromethane, chloroform and mixtures thereof.
 24. The process according to claim 17, wherein the mixture in step (x) is dried at about 25° C.-50° C.
 25. The process according to claim 17, wherein the mixture in step (x) is dried for about 5-48 hours.
 26. A process of purifying N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester comprising: (i) filtering a solution containing N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester in methanol; (ii) removing methanol under reduced pressure to get crude N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester; (iii) stirring in an aqueous organic solvent to create a slurry; (iv) filtering the slurry; (v) washing the slurry with the aqueous organic solvent; and (vi) drying the slurry to get pure N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester.
 27. The process according to claim 26, wherein the methanol is removed at a pressure of about 10-250 mm Hg.
 28. The process according to claim 26, wherein the methanol is removed at a temperature of about 20° C.-50° C.
 29. The process according to claim 24, wherein the aqueous organic solvent added in step (iii) is selected from the group consisting of water and an organic solvent selected from the group consisting of tert butyl methyl ether, ethyl acetate, diethyl ether, methyl acetate, butyl acetate, propyl acetate, isopropyl acetate, cyclohexane, acetone, hexane, heptane, pentane, acetonitrile, methanol, ethanol, isopropanol, butanol, dichloromethane, chloroform and mixtures thereof.
 30. The process according to claim 26, wherein the aqueous organic solvent added in step (iii) is a mixture of water and an organic solvent in a ratio of about 1:1 by volume.
 31. The process according to claim 26, wherein the mixture in step (iii) is stirred for about 24 hours.
 32. The process according to claim 26, wherein the mixture in step (iii) is stirred at about 15° C.-45° C.
 33. The process according to claim 26, wherein the aqueous organic solvent used to wash the slurry in step (v) is a mixture of water and an organic solvent in a ratio of about 1:1 by volume.
 34. The process according to claim 26, wherein the mixture in step (vi) is dried for about 5-48 hours.
 35. The process according to claim 26, wherein the mixture in step (vi) is dried at about 25° C.-50° C. 